Existing optical communications networks comprise a large number of system elements connected by optical fibres. A defect in such a network resulting in a loss of signal can be detected at a receiver by digital processing of the received signal bits after photoelectric conversion as described for example in U.S. Pat. No. 5,563,893 and U.S. Pat. No. 5,572,515. The majority of modern communications networks are synchronous communications systems conforming to frame format specifications such as those defined in accordance with the SONET standard specified by the American National Standards Institute (T1.105-199X, "American National Standard for Telecommunications--Digital Hierarchy--Optical Interface Rates and Formats Specification (SONET)").
It is anticipated that the next generation of optical networks will rely increasingly on system elements such as cross connects (optical switches) which are all optical in that they function without conversion to the electrical domain. There is also a tendency towards propagation at higher bit rates of 10 Gbit/s or higher making it increasingly more attractive to rely upon all optical processing, particularly where an element is remotely located, in view of the complexity and cost of ultra high speed electronic circuits.
As noted by Mathias Bischoff et al in IEEE Communications Magazine, November 1996, "Operation and Maintenance For An All Optical Transport Network", an important aspect of operation and maintenance of such all optical transport networks is likely to be the provision of optical failure detectors at various parts of the network, to enable defects in the network to be rapidly identified and remedial action taken appropriately. It is proposed for example that loss of signal may be detected by measurement of optical channel power. Channel power is however an unreliable indicator of signal presence because, in the absence of a data carrying signal, optical amplifiers and repeaters in a span of the network will tend to compensate by amplifying random noise, thereby fully or partially restoring the level of optical channel power in the absence of the data carrying signal. It is alternatively proposed that loss of signal may be detected as a result of the decoding process since the decoding apparatus will be unable to maintain synchronisation with a frame structure of received signals when a loss of signal condition exists. Other forms of signal degradation may also be detected at the decoding stage by measurement of signal to noise ratios or analysis of eye pattern statistics. Such decoding however requires conversion to the electrical domain and processing at the full bit rate of the data transmission.
Other known methods of monitoring the performance of optical communications systems include the modification of transmitted signals by the addition of a signature which can subsequently be traced through the system, as for example described in U.S. Pat. No. 5,513,029 which proposes the use of low frequency dither signals. The use of such dither or other forms of tracing signatures however may not be acceptable in a highly complex network accessed by many users and it would be preferable to avoid the need to modify the content of the optical signals carried by the system.
There remains a need to provide for loss of signal detection in such optical networks in a manner which is relatively simple to implement and cost-effective, thereby enabling loss of signal detection to be implemented at a large number of distributed monitoring locations of the network.